Pipe thermally insulated by an elastomeric material and manufacturing method

ABSTRACT

The present invention relates to a method intended for heat insulation of a pipe ( 1 ), comprising the following successive stages of depositing, on the outer surface of the pipe moving longitudinally, a thickness ( 3 ) of an insulating material made of elastomer having an elastic type behavior, substantially without irreversible plastic deformation, forming at least one circumferential slot ( 4 ) in the thickness of the material substantially orthogonally to the axis ( 2 ) of the pipe, and coating the insulating material thickness with a sealed sheath. In a variant, the slot is machined in the thickness of the insulant. The invention further relates to a heat-insulated pipe.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional application of U.S. Ser. No.09/440,283, filed Nov. 15, 1999, now U.S. Pat. No. 6,461,554.

FIELD OF THE INVENTION

The present invention relates to a heat-insulated flexible pipe used forexample for carrying effluents produced by oilwells, or for shipping orland carriage of liquid requiring thermal insulation.

The problem of thermal insulation of subsea petroleum production pipesarises in particular for reservoirs whose effluents, subjected to fastcooling due to the sea bottom temperature and to the thickness of thewater depth, undergo physico-chemical phenomena that disturb their flowin the pipe. Hydrate formation, paraffin, asphaltene deposition or oilgelation may notably occur.

The term flexible pipes refers here to pipes consisting of polymerlayers and metal reinforcing armours, and also to wound, then unwoundmetal tubes. In fact, the problem of thermal insulation is the same forboth types of pipe: the thickness of the insulating material must notstiffen the pipe so that it can be used in the same way as a pipewithout an insulant. In other words, the “flexibility” or the “rigidity”of the pipe must be substantially identical, with or without aninsulant.

BACKGROUND OF THE INVENTION

Document EP-4,006,689 describes a flexible pipe thermally insulated byhelical winding of strips made of expanded plastic. These relativelythin strips are wound by elastic deformation around the core of theflexible pipe. The insulating material selected should therefore allowimplementation of this winding operation, which is furthermore quiteextensive since several layers are necessary, while having a sufficientcharacteristic as regards heat insulation and mechanical strength underdifficult hydrostatic pressure conditions. However, this solution is notappropriate with a high outside pressure because the wound strips do notwithstand high compressive stresses.

Rigid plastic or elastomeric materials are well-suited for insulationand mechanical strength, but they pose a flexibility problem with greatthicknesses. Elastomer type materials undergo no plastic deformation,they however stiffen the pipe too much when their thickness is great,i.e. of the order of several centimeters.

SUMMARY OF THE INVENTION

The present invention thus relates to a method intended for heatinsulation of a flexible pipe according to the definition given here,comprising the following successive stages:

depositing, on the outer surface of the pipe moving longitudinally, athickness of insulating material made of elastomer, having an elasticbehaviour,

forming at least one circumferential slot in said thickness of thematerial in relation to the axis of the pipe so as to restore or topreserve the flexibility of the pipe once coated with the insulatingmaterial,

coating the insulating material thickness with a sealed sheath.

In a first variant, the slot can be made by machining.

In a second variant, the slot can be made as the material is depositedon the pipe, for example by winding strips with a gap corresponding tothe width of the slot.

According to the invention, the slot can be helical, with a determinedwidth and pitch according to the allowable bending radius of theflexible pipe.

In a variant, the slot can consist of a succession of circular slotshaving a determined width and pitch according to the allowable bendingradius of the flexible pipe. The term pitch refers here to the distancebetween two slots on a generatrix of the pipe.

An extruding head can be concentric to the pipe.

In a variant, an extruding head can be arranged laterally to the pipeand revolve in relation to the pipe so as to deposit said material inthe form of a helical strip.

The insulating material can be deposited in several successive layers soas to obtain great insulating material thicknesses.

In the case where the operation is carried out with several layers, eachlayer can comprise machined slots.

The invention also relates to a heat-insulated flexible pipe comprisingat least one layer of an insulating material made of elastomer, havingan elastic behaviour and deposited on the outside of the pipe. The layercomprises means for restoring the flexibility of the insulated pipe inthe form of at least one circumferential slot and it is coated with asealed sheath.

In the pipe, the width and the pitch of the slots can be calculatedaccording to the allowable bending radius of the pipe.

The insulating material of the pipe is sufficiently elastic to fill inthe slot when the pipe is subjected to the hydrostatic pressure.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the present invention will be clearfrom reading the description hereafter of non limitative examples,illustrated by the accompanying drawings wherein:

FIGS. 1a and 1 b show the principle of the invention,

FIGS. 2a to 2 e show different manufacturing method variants for thepipe according to the invention,

FIG. 3 shows another variant of the invention.

DETAILED DESCRIPTION

FIGS. 1a and 1 b show, in longitudinal section, a flexible pipe 1 ofallowable winding radius rm and of longitudinal axis 2. The diameter ofthe insulated pipe is D. The elongation ratio proportion of the outerfiber can be calculated as follows:

A=D/2 rm (%), assuming that the flexurally neutral fiber is situated onthe longitudinal axis 2 of the pipe.

According to the pitch p between two successive slots 4 formed or cut inthe layer 3 of insulating material, the width of the slots e and thepitch p can be estimated by taking into account an elongation proportionvalue generally allowed for high-pressure pipes, generally between 5 and8%.

For example, for a 7% elongation and a slot of width e=3 mm, the pitch pbetween the slots should be about (3/7)×100=43.

This means that, in this case, circumferential slots 4 should be spacedout by about 50 mm or, if the hollow consists of a helical slot, thespiral pitch should be about 50 mm. FIG. 1b diagrammatically shows therole of the slots in restoring the flexibility of the pipe despite therigidity of the insulating material in the case where the outsidepressure applied to sealed sheath 10 does not lead to compressivedeformation of the insulating material.

The invention uses here an elastomeric insulating material whose elasticmodulus is much lower than that of a plastic material, for example below100 MPa, and often of the order of some Mega Pascals. The materialselected has a highly elastic behaviour for extensive deformation(rubber elasticity).

When handling the pipe under low hydrostatic pressure (at the surface orin shallow water), the insulating material is not deformed or not much,and the slots allow a good pipe flexibility to be preserved.

When the pressure increases, the material under external compressiontransmitted by sealed sheath 10 is deformed and the slots areprogressively filled. The flexibility of the pipe is then decreasedsince there is no slot effect any more, but the elastic modulus of theelastomer being low, the global rigidity remains lower than in the caseof insulation with a material having a higher modulus.

Under deep water (high pressure), the slots being filled, the layer ofelastomeric material becomes nearly incompressible (the hydrostatic bulkmodulus being of the order of 1000 to 2000 MPa). The thickness of thisinsulating layer is thus not altered any more. Furthermore, if the outersheath breaks, insulation cannot be modified by the water. Filling ofthe slots also has the advantage of eliminating a possible problem ofcreep of the outer sheath in the slots under the effect of the outsidehydrostatic pressure. In fact, this problem can persist with rigidinsulating materials separated by slots that are not filled.

The order of magnitude of the compressive deformation of the insulatingmaterial can be estimated by means of FIG. 1a.

The outside diameter of the insulant decreases from D to D−ΔD and thewidth between the slots increases from L to L+ΔL. The diameter d of thepipe is considered not to vary.

The volume V is V=π/4(D²-d²)L.

The material is considered to be incompressible, therefore ΔV=0.

We thus have ΔD/D=−(ΔL/L)×(D²-d²)/2D².

Numerical applications:

with: d=30 cm, D=40 cm and ΔL/L=0.05, we have ΔD/D≈0.01;

with: d=30 cm, D=44 cm and ΔL/L=0.05, we have ΔD/D≈0.0134.

The outer sheath should therefore withstand a diameter decrease of about1%, i.e. a compression of some per cents, which is allowable for thepipes according to the present invention.

FIGS. 1a and 1 b show, in dotted lines, the theoretical shape(dimensions have been exaggerated for visibility reasons) taken by theelastic insulant under compression in the substantially rectilinearposition of the pipe and in a bent position.

The elastic behaviour of these elastomers is reversible, i.e. when apipe thus insulated is lifted from the sea bottom, the slots are clearedas a result of the decompression of the material (this reversibility ismore or less partial according to the type of elastomer used, anelastomer having a low remanent deformation under compression musttherefore be selected). After decompression, the flexibility of the pipeis restored.

The elastomeric materials can be cross-linked type (cured) orthermoplastic type materials. In general, chemically cross-linkedelastomers have a better resilience than thermoplastic materials. Thelatter are however easier to use.

The thermal conductivity of these materials is of course higher thanthat of lighter materials (foams, . . . ). It ranges most often between0.15 and 0.40 W/mK. But foams do not withstand pressures above severalMega Pascals. The heat insulation characteristics can however beimproved by using an elastomer containing hollow balls that mustwithstand the desired hydrostatic pressure. The hollow glassmicrospheres used in syntactic materials are for example well-known.

FIGS. 2a to 2 e diagrammatically show manufacturing means according tothe invention.

In the present invention, the term circumferential slot designatescircular (annular) slots, continuous or discontinuous (i.e. the slot iscut only in a portion of the circumference), slots arranged according toa continuous or discontinuous helix.

Reference number 1 designates the flexible pipe to be insulated by meansof the layer(s) 3 of insulating material extruded by an extrusion die 5supplied with fluidized material by extruder 6. The layer of insulatingmaterial is hardened by fast cooling in cooling means 7. A cuttingsystem 8 revolves around the pipe to machine the slots. The systemconsists of one or more saw blades driven by a motor. The cutting systemrevolves around the pipe to form slots over part or all of the peripheryof the pipe. The system can be controlled according to the forwardmotion of the pipe so as to make one or more helical slots, or it can bestationary in relation to the pipe during the machining procedure in thecase of a continuous circular slot and of a partly circular slot. In thelatter variant, means for controlling the displacement sequence arerequired to reposition the saw in order to cut a new slot at a distanceequal to the desired pitch, considering the longitudinal displacement ofthe flexible pipe as a whole. The depth of the slot is adjusted so as tobe at most less than or equal to the thickness of the extruded insulantlayer. In practice, adjustment allows a small thickness of insulant 11(FIG. 1a) to remain, which does not hinder the flexibility of theinsulated pipe but facilitates adjustment of the machining depth.

FIG. 2b shows a variant with two extruders and a single die. Consideringthe great thickness generally desired for the insulant, a secondextruder can be necessary for feeding the die.

FIG. 2c illustrates another variant where extrusion of the insulatingmaterial is performed in several stages so as to have several successivelayers allowing to obtain a relatively great final thickness. The firstlayer 3 a can be cut by means of a first machining system 8 a. Thesuccessive layers can also have their own machining system (8 b forlayer 3 b). In this variant, a layer of tape is preferably placed onlayer 3 a so as to prevent the extrusion performed by die 5 b fromfilling the slots machined in layer 3 a. Using two separate extruderscan allow layers of different materials to be deposited.

FIG. 2d shows a manufacturing variant having the same advantages asvariant 2 c, but the manufacturing device comprises a single slotmachining system.

FIG. 2e illustrates a very particular use of the manufacturing methodwhere there is no co-axial die as before, but radial extrusion openings9 a, 9 b whose function is to deposit the insulating material in theform of helically wound strips on the flexible pipe.

Of course, in this case, either pipe 1 is driven in rotation around itslongitudinal axis, or the extrusion system revolves around the pipe. Thelatter solution is difficult to implement, except if the devicecomprising the extrusion openings revolves around the pipe, the extruderitself remaining stationary and supplying the rotating die by means of arotary joint. In the case of a rotating die that deposits a striplaterally, the helical slots can be formed directly by means of aspecific forming device leaving a space corresponding to the desiredslot width.

It is clear that all the materials that can be extruded and quicklycooled in order to be machined according to the present method aresuited. Thermoplastic elastomeric materials are preferably used here.The flexible pipe comprises an extruded external sealed sheath 10 (FIG.1a).

FIG. 3 is a sectional view of the winding principle according to which astrip of elastomeric material, as defined above, is wound around thepipe 1 to be insulated. The elastic modulus of the material is such thatit can actually be wound in the form of a strip insofar as its thicknessallows to do so. In this case, it is clear that the slots are obtainedby helical deposition with a gap corresponding to the desired slotwidth. If a greater insulant thickness is desired, several strips can bewound on top of each other, as shown in the figure, with or withoutsticking.

What is claimed is:
 1. A heat-insulated flexible pipe, comprising: apipe, at least one insulating layer of an elastomeric material having anelastic behaviour provided on the outside of said pipe, at least oneslot provided in said elastomeric material on said pipe for restoringflexibility to the insulated pipe, and an external sealed sheathcovering said at least one layer of elastomeric material but not fillingsaid at least one slot.
 2. A pipe as claimed in claim 1, wherein thewidth and the pitch of said at least one slot are calculated accordingto the allowable bending radius of said pipe.
 3. A pipe as claimed inclaim 1, wherein the at least one insulating layer is an elasticmaterial which fills said at least one slot when deformed when the pipeis subjected to hydrostatic pressure.